Increasing demands placed on the driving, operating and control safety of modern motor vehicles require increased monitoring of driving states of the vehicle. For example, in vehicle transmissions having automatic or automated gear or gear ratio changes, monitoring the slippage in the transmission is part of the state of the art. Any impermissible slippage occurring in the transmission outside of gearshifts or gear ratio changes is typically determined by a simple plausibility query of the rotational speed and/or gear ratio conditions. Any impermissible slippage occurring in the transmission during a gear or gear ratio change is typically determined by monitoring the rotational speed behavior or required rotational speed and/or gear ratio tolerance ranges. In either case, typically the signals of rotational speed sensors at the transmission input and the transmission output are evaluated. An automatic transmission comprising a starter element located in the power flow, between the drive motor of the vehicle and the transmission, and configured as a torque converter or hydrodynamic clutch is typically provided with a turbine rpm sensor at the transmission input and a so-called output side rpm sensor at the transmission output. In order to be able to determine slippage in the transmission in each case measurable minimum rotational speeds are required, which are dependent on the respective available rotational speed detection system (sensor type, pulse sequence and or pulse separation). This means that the vehicle must roll at a certain minimum speed so that usable measurement signals are available for the gear and/or gear ratio monitoring process.
Particularly in the case of so-called “shift-by-wire systems”, where no mechanical connection exists between the automatic transmission and the driving position selection device of the automatic transmission disposed in the vehicle interior and operable by the driver, particularly high demands are placed on the protective and monitoring mechanisms in order to verify, in particular whether the operating commands of the driver are properly implemented or carried out in the transmission. A further complicating factor for the protective and monitoring mechanisms is that in modern automatic, multi-ratio transmissions, often times the shifting elements have multiple uses such that, in modern automatic, multi-ratio transmissions, at least one of the shifting elements thereof is shifted in a non-positive manner in different gears.
If such an automatic transmission is in the neutral position, the same must be secured in the neutral position against inadvertent torque transmission or a inadvertent frictional engagement, because the motor vehicle, having an automatic transmission of this type, would otherwise be unintentionally set in motion. In such an automatic transmission in the parking position, if an inadvertent torque connection or frictional engagement were to exist, this could result in damage to the transmission. Thus, in the previously unpublished German patent application DE 10 2006 014 947.5 by the Applicant, a method is proposed for operating an automatic multi-ratio transmission wherein, for a preset neutral position of the transmission, in order to secure the transmission from inadvertent torque connection or frictional engagement, at least one of three shifting elements of the transmission that is engaged in the forward gear or reverse gear is completely disengaged. In this way, at least during normal operation of the transmission, it is ensured that the drive motor and the drive axle of the motor vehicle are completely separated from one another when the transmission is in a neutral position.
In an automatic multi-ratio transmission which is such that during normal operation in the neutral position at least a first shifting element, which is required to establish a frictional engagement in the transmission, is completely disengaged and a second shifting element, which is likewise required to establish the frictional engagement in the transmission, is only partially pressurized—which is to say a low pressure is applied by a pressurized medium, the pressure not yet being sufficient for torque transmission—or is also completely disengaged, in the neutral position a frictional engagement may occur in the transmission in a faulty and inadvertent manner if these two shifting elements are erroneously selected or if these two shifting elements, as such, exhibit faulty behavior or a defect. A simple error accordingly does not necessarily result in the inadvertent frictional engagement in the transmission.